Electric vehicles such as electric wheelchairs include those having a drive device for driving each of left and right drive wheels individually with a motor (as disclosed, for example, in Japanese Patent No. 3432977, JP-A No. 9-296829 and JP-A No. 2000-70309).
The drive device disclosed in Japanese Patent No. 3432977 comprises a hollow hub case rotatably mounted on an axle at the center of the drive wheel, a metal closure covering an opening of the hub case, a case having enclosed therein a motor for the drive wheel and an electromagnetic brake, fixed to the wall of the hub case opposite to the closure and positioned as offset from the axle, and a reduction gear and a power controller which are housed in the hub case.
The closure is made of a metal having high thermal conductivity to produce an enhanced effect to radiate heat from the motor and the power controller. The power controller is disposed on the other side of the axle opposite to and away from the motor to thereby suppress the overheating of the controller (see, for example, Japanese Patent No. 3432977).
With the drive device disclosed in Japanese Patent No. 3432977, the motor and the electromagnetic brake are arranged on the other side of the cooling closure opposite to the hub case and positioned as offset from the axle. This arrangement invariably gives the drive device an increased diameter (about 30 to about 40 cm) and also gives an increase diameter to the wheel provided around the drive device. The increase in the wheel diameter is likely to make it difficult for the user to move from the wheelchair, for example, to a bed, renders the wheelchair itself greater in size and weight and therefore makes the wheelchair inconvenient to move indoors and to drive with a short turning radius.
The drive devices disclosed in JP-A No. 9-296829 and No. 2000-70309 comprise an electromagnetic brake for braking or holding the rotation of the drive shaft when the motor is deenergized, and a clutch mechanism for blocking the braking force or holding force. The brake is disposed on the other side of the motor opposite to the drive wheel, and the clutch mechanism is interposed between the motor and the drive wheel.
When the clutch is disengaged, the transmission of power from the motor to the drive wheel is therefore interrupted, and the motor fails to deliver its braking force to the drive wheel. Accordingly, if the user disengages the clutch mechanism in error with the electromagnetic brake actuated, for example, on a downward slope, an effective braking force as of dynamic braking fails to act on the drive wheel, possibly permitting the vehicle to move down the slope while being accelerated.